Effective number of bits can also be useful in describing the

AD’s noise performance An ideal AD converter will have

some amount of quantization noise determined by its reso-

lution which will yield an optimum SN ratio given by the

following equation

where n is the AD’s resolution in bits

The effective bits of a real AD converter therefore can be

found by

602

wave input signal will typically have a SN of 78 dB which is

equivalent to 126 effective bits

Two samplehold specifications aperture time and aperture

jitter are included in the Dynamic Characteristics table

since the ADC12451 has the ability to track and hold the

analog input voltage Aperture time is the delay for the AD

to respond to the hold command In the case of the

ADC12451

the hold command is internally generated

When the Auto-Zero function is not being used the hold

command occurs at the end of the acquisition window or

seven clock periods after the rising edge of the WR The

delay between the internally generated hold command and

the time that the ADC12451 actually holds the input signal is

the aperture time For the ADC12451 this time is typically

100 ns Aperture jitter is the change in the aperture time

from sample to sample Aperture jitter is useful in determin-

ing the maximum slew rate of the input signal for a given

accuracy For example an ADC12451 with 100 ps of aper-

ture jitter operating with a 5V reference can have an effec-

tive gain variation of about 1 LSB with an input signal whose

slew rate is 12 V ms

50 Typical Applications

Power Supply Bypassing

TLH11025 – 24

Protecting the Analog Inputs

TLH11025 – 25

Note

External protection diodes should be able to withstand the op amp current limit

16